The selling point of the Mi version, which stands for Mobile internet, is the Linux OS dressed up in nice clothes by HP. More here

On the HP Mini 1000 Mi, the company chose to bundle Ubuntu, one of the most popular Linux distributions on the market. Nevertheless, HP listened to netbook costumers regarding their experience with Linux. Hence, average users simply won’t get the awkward feel of using Linux, as they feel on the Linpus Linux on the Aspire One or on the Xandros OS on the EEE PC series. Instead, the company developed a very appealing interface, which gathers all the internet, email and multimedia tasks on its home screen.